1. Field of The Invention
The present invention relates to compositions comprising conductive particles and one or more polymers, particularly acid copolymers or ionomers which can be extruded or heat formed into films or coatings. More specifically, the heat processing of the present invention is directed to a novel, non-uniform heating method, wherein the edge portions of a material comprising acid copolymer and/or ionomer is heated 3-50 degrees Celsius hotter than the central or middle portion of the article or film during fabrication to thereby provide improved and substantially uniform surface resistivity properties.
2. Description of The Related Art
Plastics are often considered for use as electrical insulating materials, because they typically do not readily conduct electrical current and are generally rather inexpensive relative to other known insulating materials. A number of known plastics are sufficiently durable and heat resistant to provide at least some electrical insulating utility, but many such plastics are problematic due to the accumulation of electrostatic charge on the surface of the material.
Such surface charge accumulation can be undesirable for various reasons. Such materials sometimes discharge very quickly, and this can damage electronic components, or cause fires or explosions, depending upon the environment. Sudden static discharge can also be an annoyance to those using the material.
Even where sudden static discharge is not a problem, dust will typically be attracted to and will accumulate on materials carrying a static charge. Furthermore, the static charge can interfere with sensitive electronic components or devices and the like.
Resistivity can be defined as involving surface resistivity and volume resistivity. If the volume resistivity is in an appropriate range, an alternative pathway is provided through which a charge can dissipate (generally along the surface). Indeed, surface resistivity is typically the primary focus for electrostatic dissipating ("ESD") polymeric materials.
Surface resistivity is an electrical resistance measurement (typically measured in ohms per square) taken at the surface of a material at room temperature. Where the surface resistivity is less than or equal to about 10.sup.5, the composition's surface has very little insulating ability and is generally considered to be conductive. Such compositions are generally poor electrostatic dissipating polymeric materials, because the rate of bleed off is too high.
Where the surface resistivity is greater than 10.sup.12, the composition's surface is generally considered to be an insulator. In certain applications, such a composition is also poor electrostatic dissipating material, because the surface does not have the requisite amount of conductivity necessary to dissipate static charge. Typically where the surface resistivity is about 10.sup.5 to 10.sup.12, any charge contacting the surface will readily dissipate or "decay". Further information involving the evaluation of surface resistivity can be found in American Standard Test Method D257.
Acid copolymer resins are a well-known class of polymers containing up to about 30 weight percent organic acid groups which are attached to a hydrocarbon or perfluorinated polymer chain. Ionomers are derived from these acid copolymer resins by partial neutralization of the acid groups with metal ions, such as zinc, sodium, or magnesium.
Acid copolymers and ionomers generally have surface resistivities greater than 10.sup.12, and therefore these materials are generally not suitable for high performance ESD uses.
Static charge decay rates measure the ability of an electrostatic dissipating material to dissipate charge. A 90% decay time as used herein is measured at about 15% relative humidity and at ambient temperature as follows: A 5 kilovolt charge is placed upon the material and the amount of time (in seconds) for the charge to decrease to 500 volts is measured. A 99% decay time is measured substantially as for the 90% decay time, except that the amount of time measured is for the charge to dissipate to 50 Volts.
Many electrostatic dissipating materials generally found in the art have a 90% decay time of greater than about 3 seconds and a 99% decay time of greater than about 5 seconds. However, the National Fire Protection Association standard (NFPA Code 56A) requires 0.5 seconds as an upper limit for a 90% decay time, and the U.S. Military Standard (MIL-81705C) requires 2.0 seconds as an upper limit for a 99% decay time. Due to high surface resistivities, acid copolymer and ionomer compositions generally cannot meet such rigorous criteria as NFPA Code 556A or MIL-81705C.
Attempts have been made to coat an electrostatic dissipative material onto an insulating plastic to reduce the accumulation of static charge. However, surface applications have been problematic due to long term adhesion requirements and interference with surface properties.
Conventional low molecular weight organic electrostatic additives typically work well only in the presence of high relative humidity. Such additives typically must bloom to the surface after blending or mixing to provide electrostatic dissipative performance, and such blooming may not always be consistent. These additives may also cause thermal stability problems during processing or may cause physical properties of the produced film or coating to deteriorate. Such additives can also wash away or abrade from the surface.
High molecular weight (polymeric) electrostatic dissipating agents for plastic are known, but they can be expensive, can undesirably alter the properties of the material and can be difficult to blend or alloy into a polymer material.